The members of the cyclin-dependent kinase (CDK) family play critical regulatory roles in cell proliferation. There are currently twenty known mammalian CDKs. While CDK7-CDK13 have been linked to transcription, only CDK1, 2, 4, and 6 show demonstrable association with the cell cycle.
Unique among the mammalian CDKs, CDK7 has consolidated kinase activities, regulating both the cell cycle and transcription. In the cytosol, CDK7 exists as a heterotrimeric complex and is believed to function as a CDK1/2-activating kinase (CAK), whereby phosphorylation of conserved residues in CDK1/2 by CDK7 is required for full catalytic CDK activity and cell cycle progression (Desai et al., “Effects of phosphorylation by CAK on cyclin binding by CDC2 and CDK2.” Mol. Cell Biol. 15, 345-350 (1995); Kaldis et al., “Analysis of CAK activities from human cells.” Eur. J. Biochem. 267, 4213-4221 (2000); Larochelle et al., “Requirements for CDK7 in the assembly of CDK1/cyclin B and activation of CDK2 revealed by chemical genetics in human cells.” Mol. Cell 25, 839-850 (2007)). In the nucleus, CDK7 forms the kinase core of the RNA polymerase (RNAP) II general transcription factor complex and is charged with phosphorylating the C-terminal domain (CTD) of RNAP II, a requisite step in gene transcriptional initiation (Serizawa. et al., “Association of CDK-activating kinase subunits with transcription factor TFIIH.” Nature 374, 280-282 (1995); Shiekhattar et al., “CDK-activating kinase complex is a component of human transcription factor TFIIH.” Nature 374, 283-287 (1995); Drapkin et al., “Human cyclin-dependent kinase-activating kinase exists in three distinct complexes.” Proc. Natl. Acad. Sci. U.S.A. 93, 6488-6493 (1996); Liu. et al., “Two cyclin-dependent kinases promote RNA polymerase II transcription and formation of the scaffold complex.” Mol. Cell Biol. 24, 1721-1735 (2004); Akhtar et al., “TFIIH kinase places bivalent marks on the carboxy-terminal domain of RNA polymerase II.” Mol. Cell 34, 387-393 (2009); Glover-Cutter et al., “TFIIH-associated CDK7 kinase functions in phosphorylation of C-terminal domain Ser7 residues, promoter-proximal pausing, and termination by RNA polymerase II.” Mol. Cell Biol. 29, 5455-5464 (2009)). Together, the two functions of CDK7, i.e., CAK and CTD phosphorylation, support critical facets of cellular proliferation, cell cycling, and transcription.
CDK12 and CDK13 were identified in cDNA screens for cell cycle regulators. Because their cyclin partners were not yet known, they were initially named CRKRS and CDC2L5 (Ko et al., J. Cell Sci., 2001, 114, 2591-2603; Marques et al., Biochem Biophys Res Commun., 2000, 279(3):832-837), respectively. They were found to be 1490- and 1512-amino acid proteins, respectively, with a conserved central CTD kinase domain and degenerate RS domains identified in their N- and C-terminal regions (Even et al., J Cell Biochem., 2006, 99(3), 890-904).
Evidence has shown CDK12 and CDK13 play an important role in cancer development. A comprehensive genomic approach identified CDK12 to be one of the most frequently somatically mutated genes in high-grade serous ovarian cancer, the most fatal form of the disease (Erratum, Nature, 2011, 474(7353), 609-615). Several identified point mutations in the kinase domain point to the critical importance of the kinase activity of CDK12 for the development/progression of this disease. CDK12 has also been found to contribute to the development of breast cancer. Notably, CDK12 is located on chromosome 17, within the 17q21 locus that contains several candidate genes for breast cancer susceptibility (Kauraniemi et al., Cancer Res., 2001, 61(22), 8235-8240), and it is co-amplified with the tyrosine kinase receptor ERBB2, a protein amplified and overexpressed in about 20% of breast tumors. Gene fusion between CDK12 and ERBB2 was also detected in gastric cancer (Zang et al., Cancer Res., 2011, 71(1), 29-39). CDK12 is also implicated in the modification of tamoxifen sensitivity in estrogen-positive breast cancer via the modulation of the mitogen-activated protein kinase pathway (Iorns et al., Carcinogenesis, 2009, 30(10):1696-1701).
Due to the important regulatory functions of kinases, such as CDK7, CDK12, and CDK13, in cell cycle control, cell proliferation, differentiation, and apoptosis, it is important to develop modulators of the activities of these kinases, including selective modulators, for use as research tools as well as therapeutic agents in the treatment of diseases.